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Sex Determination/Transcript
Transcript Text reads: The Mysteries of Life with Tim and Moby A boy, Tim, and a robot, Moby, are looking into the bathroom mirror. Tim rubs a finger on his chin and across his upper lip. TIM: I think there are a few moustache hairs coming in. MOBY: Beep. TIM: What? Moby taps Tim on the head and hands him a large lollipop. Tim grumbles and reads from a typed letter. TIM: Hi Tim and Moby, How come some babies are male and others are female? From, Devon. TIM: Oh. Well, like most physical traits, sex is determined by your genes, or DNA. DNA stands for deoxyribonucleic acid. An animation shows a strand of DNA. TIM: You can think of DNA as a blueprint for your body. It controls stuff like hair color and height, but also day-to-day functions, like cell division and immune response. Images show the outline of a human body, surrounded by symbols representing the traits and processes Tim describes. TIM: DNA is arranged in long, twisty strands called chromosomes. An image shows a chromosome. TIM: Almost every cell in your body contains 23 pairs of chromosomes. One chromosome in each pair comes from your mother, and the other comes from your father. An animation shows 23 pairs of chromosomes, along with images of Tim's mother and father. TIM: Scientists number the different pairs to keep track of which genes are in charge of which functions. The images of Tim's parents disappear, and a number appears beneath each pair of chromosomes. They are numbered from one to twenty-three. TIM: The twenty-third pair of chromosomes determines your sex. A circle appears around the twenty-third pair of chromosomes. TIM: There are two basic chromosome types that can make up the pair, X and Y. Images show an X chromosome and a Y chromosome. TIM: A Y chromosome causes the body to produce testosterone and develop male body parts. MOBY: Beep. TIM: Right, so in males, the twenty-third pair is made up of an X and a Y chromosome. An animation shows an X and a Y chromosome, along with the symbol for a male. TIM: Whereas in females, it's made up of two X-es. An animation shows two X chromosomes, along with the symbol for a female. MOBY: Beep. TIM: Well, being born male or female pretty much comes down to chance. See, everybody comes from the combination of a sperm cell from the father, and an egg cell from the mother. An animation shows a sperm cell and an egg cell. TIM: Unlike all the other cells in your body, sperm and egg cells are haploid, meaning they contain 23 individual chromosomes, not 23 pairs. Images show the 23 chromosomes of a sperm cell and the 23 chromosomes of an egg cell. TIM: When a sperm cell fertilizes an egg, the single sets of chromosomes from each cell merge, forming a complete set of 23 pairs. An animation shows the two sets of single chromosomes merging and creating 23 pairs of chromosomes. TIM: Because females only have X chromosomes, all their egg cells contain an X for the twenty-third chromosome. An animation shows several egg cells, each with an X chromosome. TIM: Males have both X and Y chromosomes. An animation shows four swimming sperm cells. Two have an X chromosome, and two have a Y chromosome. TIM: That means a sperm cell has an equal chance of containing an X or a Y. The sperm cells close in on a single egg cell. TIM: So, if a Y sperm fertilizes an egg, you'll get an XY baby, or a male. A sperm with a Y chromosome fertilizes the egg. An XY chromosome pair forms in the egg. TIM: If an X sperm fertilizes an egg, you'll get an XX baby, or a female. The Y chromosome disappears from the egg. An X sperm fertilizes it. An XX chromosome pair forms in the egg. MOBY: Beep. TIM: It works pretty much the same way in every organism that reproduces sexually. MOBY: Beep. TIM: Uh yeah, actually. It's pretty rare, but people can be born with uncertain sex traits. An image of a human silhouette appears. A question mark flashes in the middle of the silhouette. TIM: A person with male chromosomes can have some female traits, and someone with female chromosomes can have male traits. It also happens when babies inherit an unusual number of sex chromosomes, like one or three. An animation illustrates the situations Tim describes. MOBY: Beep. TIM: Yeah, I'd say so. MOBY: Beep. TIM: Um, you're male? I think. Aren't you? Moby shrugs. Category:BrainPOP Transcripts